Re: Intermittent storage (dm-crypt?) freeze - regression 6.4->6.5

[Hannes Reinecke <hare@suse.de>]

On 11/1/23 04:24, Ming Lei wrote:
> On Wed, Nov 01, 2023 at 03:14:22AM +0100, Marek Marczykowski-Górecki wrote:
>> On Wed, Nov 01, 2023 at 09:27:24AM +0800, Ming Lei wrote:
>>> On Tue, Oct 31, 2023 at 11:42 PM Marek Marczykowski-Górecki
>>> <marmarek@invisiblethingslab.com> wrote:
>>>>
>>>> On Tue, Oct 31, 2023 at 03:01:36PM +0100, Jan Kara wrote:
>>>>> On Tue 31-10-23 04:48:44, Marek Marczykowski-Górecki wrote:
>>>>>> Then tried:
>>>>>>   - PAGE_ALLOC_COSTLY_ORDER=4, order=4 - cannot reproduce,
>>>>>>   - PAGE_ALLOC_COSTLY_ORDER=4, order=5 - cannot reproduce,
>>>>>>   - PAGE_ALLOC_COSTLY_ORDER=4, order=6 - freeze rather quickly
>>>>>>
>>>>>> I've retried the PAGE_ALLOC_COSTLY_ORDER=4,order=5 case several times
>>>>>> and I can't reproduce the issue there. I'm confused...
>>>>>
>>>>> And this kind of confirms that allocations > PAGE_ALLOC_COSTLY_ORDER
>>>>> causing hangs is most likely just a coincidence. Rather something either in
>>>>> the block layer or in the storage driver has problems with handling bios
>>>>> with sufficiently high order pages attached. This is going to be a bit
>>>>> painful to debug I'm afraid. How long does it take for you trigger the
>>>>> hang? I'm asking to get rough estimate how heavy tracing we can afford so
>>>>> that we don't overwhelm the system...
>>>>
>>>> Sometimes it freezes just after logging in, but in worst case it takes
>>>> me about 10min of more or less `tar xz` + `dd`.
>>>
>>> blk-mq debugfs is usually helpful for hang issue in block layer or
>>> underlying drivers:
>>>
>>> (cd /sys/kernel/debug/block && find . -type f -exec grep -aH . {} \;)
>>>
>>> BTW,  you can just collect logs of the exact disks if you know what
>>> are behind dm-crypt,
>>> which can be figured out by `lsblk`, and it has to be collected after
>>> the hang is triggered.
>>
>> dm-crypt lives on the nvme disk, this is what I collected when it
>> hanged:
>>
> ...
>> nvme0n1/hctx4/cpu4/default_rq_list:000000000d41998f {.op=READ, .cmd_flags=, .rq_flags=IO_STAT, .state=idle, .tag=65, .internal_tag=-1}
>> nvme0n1/hctx4/cpu4/default_rq_list:00000000d0d04ed2 {.op=READ, .cmd_flags=, .rq_flags=IO_STAT, .state=idle, .tag=70, .internal_tag=-1}
> 
> Two requests stays in sw queue, but not related with this issue.
> 
>> nvme0n1/hctx4/type:default
>> nvme0n1/hctx4/dispatch_busy:9
> 
> non-zero dispatch_busy means BLK_STS_RESOURCE is returned from
> nvme_queue_rq() recently and mostly.
> 
>> nvme0n1/hctx4/active:0
>> nvme0n1/hctx4/run:20290468
> 
> ...
> 
>> nvme0n1/hctx4/tags:nr_tags=1023
>> nvme0n1/hctx4/tags:nr_reserved_tags=0
>> nvme0n1/hctx4/tags:active_queues=0
>> nvme0n1/hctx4/tags:bitmap_tags:
>> nvme0n1/hctx4/tags:depth=1023
>> nvme0n1/hctx4/tags:busy=3
> 
> Just three requests in-flight, two are in sw queue, another is in hctx->dispatch.
> 
> ...
> 
>> nvme0n1/hctx4/dispatch:00000000b335fa89 {.op=WRITE, .cmd_flags=NOMERGE, .rq_flags=DONTPREP|IO_STAT, .state=idle, .tag=78, .internal_tag=-1}
>> nvme0n1/hctx4/flags:alloc_policy=FIFO SHOULD_MERGE
>> nvme0n1/hctx4/state:SCHED_RESTART
> 
> The request staying in hctx->dispatch can't move on, and nvme_queue_rq()
> returns -BLK_STS_RESOURCE constantly, and you can verify with
> the following bpftrace when the hang is triggered:
> 
> 	bpftrace -e 'kretfunc:nvme_queue_rq  { @[retval, kstack]=count() }'
> 
> It is very likely that memory allocation inside nvme_queue_rq()
> can't be done successfully, then blk-mq just have to retry by calling
> nvme_queue_rq() on the above request.
> 
And that is something I've been wondering (for quite some time now):
What _is_ the appropriate error handling for -ENOMEM?
At this time, we assume it to be a retryable error and re-run the queue
in the hope that things will sort itself out.
But if they don't we're stuck.
Can we somehow figure out if we make progress during submission, and (at 
least) issue a warning once we detect a stall?

Cheers,

Hannes
-- 
Dr. Hannes Reinecke                Kernel Storage Architect
hare@suse.de                              +49 911 74053 688
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